When cured, silicone gel compositions can form silicone gels which have good stress-relieving characteristics, electrical characteristics, heat resistance and weather resistance. Accordingly, such silicone gel compositions are widely used as sealing agents and filling agents in electrical and electronic parts (see Japanese Patent Application Kokai No. 9-204259, Japanese Patent Application Kokai No. 61-48945, and Japanese Patent Application Kokai No. 62-104145). Furthermore, examples of such silicone gel compositions include silicone gel compositions which consist of a vinyl-group-containing organopolysiloxane which has a branched structure and which has a viscosity of 10 to 10,000 centistokes at 25.degree. C., a linear organopolysiloxane which contains hydrogen atoms bonded to silicon atoms, and a hydrosilylation reaction catalyst (see Japanese Patent Application Kokai No. 48-17847); silicone gel compositions which consist of a vinyl-group-containing organopolysiloxane which has a branched structure, an organopolysiloxane which contains hydrogen atoms bonded to silicon atoms, and a hydrosilylation reaction catalyst (see Japanese Patent Application Kokai No. 58-7452); and silicone gel compositions which consist of a vinyl-group-containing organopolysiloxane which has a branched structure and which has a viscosity of 20 to 10,000 centistokes at 25.degree. C., a linear organopolysiloxane in which both ends of the molecular chains are closed by vinyl groups, a linear organopolysiloxane which contains hydrogen atoms bonded to silicon atoms only at both ends of the molecular chains, and a hydrosilylation reaction catalyst (see Japanese Patent Application Kokoku No. 3-19269).
However, when electrical or electronic parts which have a complicated structure are sealed or filled by means of such silicone gel compositions, gas bubbles and cracks are formed in the sealing silicone gel as a result of the heat cycle, so that the reliability of the electrical or electronic parts drops. Furthermore, it has been found that the formation of gas bubbles and cracks in silicone gels sealing or filling such electrical or electronic parts is conspicuous in the case of electrical or electronic parts in which there are narrow gaps between adjacent electrodes, adjacent electrical elements or electrical elements and packages, etc., and electrical or electronic parts in which it is difficult for such structures to conform to the expansion or contraction of the silicone gel.